Among the more important discoveries in cancer is the identification of driver mutations that create opportunities to deliver ?personalized medicine?. In this regard, non-small-cell lung cancer (NSCLC) is one of the success stories in cancer, with at least seven genomic mutations identified that can be specifically targeted by drugs. By contrast, small-cell lung cancer (SCLC), which represents 15% of all lung carcinomas, has seen only minor improvements in its standard of care over the past three decades and still relies on traditional chemotherapy as its primary treatment option due to extensive metastasis at initial diagnosis. However, a distinct subgroup of SCLC patients, ~20%, have so-called primary chemo-refractory disease and demonstrate little or no response to initial chemotherapy. Chemo-refractory SCLC patients experience much shorter survival because of minimally effective second-line agents. Thus, among the key challenges in SCLC are to 1) predict upfront which patients will not respond to chemotherapeutic agents so that 2) we can develop new therapies with efficacy in this chemo- refractory subgroup. In this regard, we have an ongoing, retrospective study of SCLC patients that uses targeted exon sequencing to identify genetic mutations in tumors that negatively impact patient outcome. In our most recent analysis we surprisingly found that patients with no mutation of tumor RB1 (i.e., wt RB1) demonstrated significantly increased chemo-refractory disease and decreased overall survival compared to those with mutated RB1. Further experiments demonstrated that RB1 protein was expressed by >50% of SCLC cell lines annotated as having wt RB1 and that cell lines expressing RB1 were specifically sensitive to CDK4/6 inhibitors. Thus, we hypothesize that RB1 mutation status and/or protein expression may identify chemo-refractory patients in SCLC and may also predict sensitivity to CDK4/6 inhibitors. This hypothesis will be tested in two Specific Aims: (1) To determine whether RB1 mutation status and/or protein expression identifies chemo-refractory patients in SCLC. We will use our database of >650 SCLC patients to select a validation cohort of tumor specimens for paired RB1 gene sequencing and protein IHC expression analyses. RB1 mutation status and protein expression will be correlated with each other as well as with the principal clinical outcomes of chemo-response and survival. (2) To determine whether RB1 mutation status and/or expression predicts sensitivity to CDK4/6 inhibitors in models of SCLC. We propose that SCLC cells with wt RB1 expression likely demonstrate functional RB1 signaling and will therefore be sensitive to CDK4/6 inhibitors. This will be examined in SCLC cell lines with variable RB1 expression, as well as in wt RB1 cells after RB1 knockdown. The mutation and expression of RB1 will also be determined in several SCLC PDX models and correlated with chemo-response and CDK4/6 inhibitor sensitivity. The innovation of our proposal is that it seeks to provide a molecular basis to explain chemo-refractory SCLC. Although our hypothesis on chemo-refractory SCLC is at odds with the wide-held belief that RB1 loss is pivotal in the genesis of SCLC; if true, it could represent a first step toward a more ?personalized? therapeutic approach for this cancer, as it would predict potential sensitivity to CDK4/6 inhibitors. This finding could be immediately translated to patients as CDK4/6 inhibitors are already in clinical trials for several tumor types.